An idiom is an expression whose meaning cannot be derived from the usual meaning of its constituents. As such, idioms present a special learning problem for non-native speakers of English, especially learners of English as foreign language (EFL). Understanding of idiomatic expressions can be important, for example, in academic settings, where presentation of ideas often involves figurative language. Even more encompassing is the notion that natural use of idioms can overtly demonstrate participation in a realm of shared cultural knowledge and interests, and so to help a learner gain social acceptance.
A long line of research has focused on EFL students strategies for comprehending idioms. Investigations of the processes of idiom comprehension by non-native English speakers from a variety of backgrounds found that subjects used a variety of strategies for comprehension. Investigations of avoidance of English idioms by EFL university students, using a fill-in translation test, found that lower English proficiency was associated with greater avoidance of English idioms. Investigations of knowledge of 50 idioms collected from the lists of frequently used English idioms found poor idiomatic competence among EFL students in Vietnam. It is known that multiple factors contribute to figurative competency, such as learners' proficiency levels, types of idioms, learners' vocabulary knowledge, similarity of idioms between L2 and L1.
Researchers have also looked at figurative language that EFL learners encounter in their educational environments and materials (e.g. textbooks, lectures, etc.). This line of research is often related to developing teaching and reference materials for language learners. A corpus-based study of the spoken American English idioms used most frequently by college students learning American English involved a close concordance search and analysis of the idioms used in three contemporary transcribed spoken American English corpora, and provided suggestions for improving the development of idiom teaching and reference materials. Investigations of the non-native speakers of English experience when encountering metaphors in British university lectures showed a range of difficulties, including nonunderstanding (failure to interpret an expression) and misunderstanding (getting an incorrect interpretation).
A complementary line of research focuses on the EFL students' use of metaphors and idioms in their linguistics productions. It has been claimed that accurate and appropriate use of idioms is a strong distinguishing mark of the native-like command of the language and might be a reliable measure of the proficiency of foreign learners. Recently, analysis of the use of metaphors in 200 exam essays written by EFL students, at different levels of English proficiency, found that metaphor use increases with proficiency level, and even suggested that descriptors for metaphor use could be integrated in the rating scales for writing. Another recent investigation of the use of metaphors in 116 test-takers argumentative essays found moderate-to-strong correlation between the percentage of metaphorically used words in an essay and the writing quality score. Notably, both recent studies used only a small number of essays and conducted an exhaustive manual analysis of metaphoric expressions.